Precision made hollow shafts are required in the manufacture of modern turbine engines and other mechanical devices having rotating parts. In aircraft, for example, most gas turbine engines comprise multiple hollow shafts. An inner shaft is frequently used to connect a low pressure compressor or fan with a low pressure turbine. Inner shafts can also be used to connect a low pressure turbine with a power takeoff assembly. A shorter but larger diameter outer (concentric) shaft is often used in the same engine to connect a high pressure compressor with a high pressure turbine. All of these shafts must be manufactured to exact dimensions, which include interior details, for shaft cooling and lubrication systems to operate correctly. Internal seals and cavities have always presented a major measurement problem when developing highly accurate and efficient assemblies which include hollow shafts. For example, if air seals are included inside a hollow shaft, seal reliability and efficiency greeatly depends on their dimensional accuracy.
In the past is has been very difficult to accurately check dimensions inside hollow shafts and internal seals. One method of measuring internal cavities has been to fill the cavity or shaft portion with a dimensionally stable liquid material such as room-temperature vulcanizing rubber. When the material solidifies, it is cut and removed, so that it can be measured for its external size which, hopefully, equals the internal dimensions of the shaft portion or cavity. In other instances where this is not possible, the machines used to manufacture these parts are made as highly reliable as possible in order to minimize the chances of dimensional variations during manufacture.
Neither of the above methods and apparatus have proven to be entirely reliable or as accurate as direct measurement. Further, other means of ascertaining internal dimensions have proven cumbersome and slow.
In view of the above, a need exists for a measurement device that can quickly and accurately be used to map the internal dimensions of a hollow shaft, seal or other cavity.